1. Field of the Invention
The present invention relates to mechanisms for producing bi-directional step motion of a shaft from a continuously rotating uni-directional source and may be utilized in various devices such as paper tape punches and readers, card readers and punches, and the like.
2. Description of the Prior Art
Paper tape has been transported through paper tape handling equipment, such as readers and punches, by various transport mechanisms. In the prevalent paper tape handling system, the tape has a series of uniformly spaced perforations along the longitudinal axis. A sprocket wheel secured to a rotatable shaft is mounted adjacent one side of the tape. The sprocket wheel has a plurality of uniformly spaced pins extending from its periphery. A portion of the pins on the sprocket wheel engage the paper tape by extending through the perforations. The paper tape can then be transported by rotation of the shaft to which the sprocket wheel is secured. It is a requirement of most paper tape handling equipment that the tape be transported along the longitudinal axis in uniform increments. Such incremental transporting can be accomplished by incremental rotation of the shaft to which the sprocket wheel is secured. The incremental rotation is defined as a step. One of the common means to step the sprocket wheel shaft is to secure a rachet to the sprocket wheel shaft and actuate the rachet by means of an electromagnetically actuated pawl. As the rachet rotates, it causes the sprocket wheel to rotate thereby transporting the tape. A detent means, usually engaging the rachet, controls the magnitude of the angular displacement of the step by causing the rachet to cease rotating at selected positions of the rachet. While the pawl, rachet and detent have been satisfactory, they posses undesirable features relating to the acceleration and velocity profile of the paper tape as it is transported, and the uni-directional step limitation of the mechanisms. When the pawl in the prior art mechanisms is electromagnetically actuated, it abruptly impacts the rachet to initiate the step. The pawl is then pulled with increasing force by the electromagnet to cause the rachet to rapidly accelerate. At some point during the step, the detent means takes over control of the rachet from the pawl causing a discontinuity in the acceleration and velocity profile of the step. The detent means further accelerates the rachet until the detent means finds its locating position, at which point the rachet is abruptly decelerated to terminate the step motion. The non-uniform, discontinuous nature of the acceleration and velocity profile associated with the prior art pawl and rachet mechanisms can cause the tape to move out of engagement with the sprocket wheel, cause the tape engaging pins to tear the paper tape, and cause undesirable wear and fatigue of the parts.
Mechanisms are available which produce step rotation of the sprocket wheel shaft where the step has a uniform and continuous acceleration and velocity profile. Some of these mechanisms are also capable of providing step rotation in either direction. Such mechanisms, however, are characteristically complex, requiring expensive cams and other parts, and many require precise adjustments not compatible with economical equipment.
A relatively simple pawl and rachet mechanism which produces a step having a desirable acceleration and velocity profile is described in U.S. Pat. No. 3,058,362 issued to G. Perez. While the Perez mechanism overcomes the non-uniform and discontinuous acceleration and velocity limitations of the prior art pawl and rachet mechanisms, it only produces a uni-directional step and requires a clutch to interrupt an input rotation source.
The present invention overcomes the limitations of the prior art by providing a relatively simple, inexpensive tape transport mechanism which can produce step motion in either direction with the step motion having a desirable acceleration and velocity profile.